Radio transmission system



2 Sheets-Sheet 1 Dec. 15 1925'- R. BOWN RADIO TRANSMISSION SYSTEM FiledSept. 18 1924 Dea-15, 1925.

R. BWN

RADIO TRANSMISSION SYSTEM Filed Sept. 18. 1924 2 Sheets-Sheet 2 ,IN 'VENT91? If,

' v This.` inventionrelates` to signaling sys- Patented Dec. l5, 1925.

UNiTi-:o STATES l RALPH iaoWN,A on WYOMING, NEW JERSEY, AssIGNoR ToAMERICAN TELEPHONE Ann TEIiEGRAPI-I QOMPANY, A CORPORATION 'OF NEW YORK.

RADIO TRANSMISSION SYSTEM.

Application filed September 18, 1924. Seriali-No. 738,511.

:.g ing at W yomin in the-county of Essex and H St `tejlif-l`evvv ersey,have invented certainv 55Improvementsin- Radio Transmission Sysof ywhichthe following is a specifica- .tiony Atems, and more particularly toarrangements for 'controlling transmission .in systems of radiocommunication. In telephone transmission and in other kinds of signalingas' well, it is often desirable to transmit sounds which have aconsiderableran e ot volume.' .It is: also necessary that the minimum-volume transm-itted .be maintained above the noise level' offthetransmission mediumY and'that r-themaximum -volume transmitted throughthedinedium shall not be suliciently great to y overloadvacuum tubesused in the system.

Intli'e caseof a radiotelephone system the noise level isdetermined to aconsiderable extent by Static disturbances and interference due toothersignalingv stations so that the minimum'signalvolumetransmitted must 'bemaintainedl fairl high. In the Vcase of a Elong. distance Ara iotelephone system, such, for exam le, as a trans-Atlantic system, thesituation isfurthei complicated by the Wide fvariation of the over-alltransmission equiv- 'z alent-"ofthe medium during the differentperiodsofthe day. l I

\ XI't, is therefore' desirablel that some means fbe providedtolcompens'ate for the changes VS .in the `transmission equivalent dueto the periodical `variation' of .the medium, `for, in other` words, tomaintain the transmission equivalent constant. In addition, itisdesirable thatl some arrangement 'be made 4 1 wherebythe'volumerangemay be compressed f viluiing transmission through Athe, medium `andagain expanded to itsoriginalv rangek at `the receiving station.Accordingly'it is proposed by the presentrinvention torprovide acontrolling or pilot frequency whiclrmay be v'transmitted along withthevsignaL-the va-'v` 4 theover-alltransmissionj equivalent of the`systeiml maintained substantially convriations in the amplitude of thereceived pilot. frequency serving toindicate at the 1receiving stationthe variations ofthe equivalentof thev transmission medium )fromnormal.`

, stant.

, By makingy suitable adjustments in the amplifier or other apparatus atthe receiving station from time-to time in accordance with the receivedindication, the over-al1 transmission equivalent -of thecommunicationsysj tem may be maintainedsubstantially con- It is also proposed toprovide equipment at the sending station for adjustingv the vol unie ofthe transmitted signalfrom time to time to lreep the range of'volumeofthe transmitted signal Within the limits imposed by the noise leveland distortion resulting from overloading'the apparatus. In fact, fromthe standpoint of eili'ciency, it is desirablethat 65 the full power ofthe transmitter be' controlled bythe speakers voice Aat all times sothat the volume actually transmitted would in such case involve verysmall variation. This result may be obtained byv adjusting vsometransmission element inthe signaling circuit at the transmittingstationas the signaling volume varies from normal so that the` powerimpressed upon the modulator Will be substantially constant. I `It is,of course, necessary, when suchadjustments are made, that a compensatingadjustment be made at the receiving station so that if the signal at thetransmitting station is reduced .it will be correspondingly augmented'at thel 80 receiving'stati'on, and vice versa. It is there- I foreproposed ,that When any, adjustment in the signalvolume. is made at thetransmit- 'ting station a corresponding adjustment be made of the'amplitude of the pilot r kcon- 85 trolling frequency. When this is oneit is possible, by adjustingthe transmission element at'thereceivingstatiomto induce a compensating-increase in volume whenever theamplitude of the receivedpilot frequency falls below normal and toinducea compensating. decrease in volume when the received pilotfrequency attains anvalue above normal. By making these adjustments latthe receiving stationflin accord-ance with Ithefino5 .dication-of. the:pilot lfrequency, not only will 1 etant, but -the reccived signalAWill` begiven the same range of variationwof volume as 10c The.operation of the apparatus so far described is as' follows: f

A voice band incoming from the line LW passes into the transmittingchanned through the low-pass filter 10, and after being amplified by theampliier'l is impressed upon the modulator M. The carrier frequency comjponent resulting from the first step of modulation is suppressed by thebalanced arrangementfof the circuit, and the upper side band suppressedby the filterl-, thelower side band being transmitted by the filter andimpressed upon the modulatorM". This modulator, which is suppliediwith acarrier frequency of' 88,500 cycles, again suppresses the carrierfrequency and produces two-side bands, the upper of which suppressed bythe filter 17, and the lower ofwhich is trans'- "mitted through thefilter. yItwill be noted that the lower side band now corresponds to theoriginal signaling band, and thierbandA having been inverted twice, thefrequencies in the band transmitted bythe filter. 17 will be in theirnormal relat'ion,'that is,.the lowest frequencyiof the band passed willcorrespond to the lowest frequency inthe original sig-` naling band, andthe'highest frequency in the' band will similarly correspond. tothehighest frequency of the original signalA band. Therefore, the result ofstepping up ythe band by two 'steps of modulation with .suppression ofthe other side `band each time results in finallylproducingxa .bandcorresponding tothe uppersid'e band' which 'u would be produced as'theresult of :modulating the signal upon a carrier frequency of 55,500cycles,

Thesignal band thus finally produced-is impressed upon the poweramplifier PA, and upon being amplified by the amplifier y ris radiatedby the antenna 18 to the receiving i responds to the' original signaland is selected by Ithe low-passl filter .Q6 and transmitted through thelow 'frequency amplifier Q'Tto thelineLE. 'y

It will thus be seen that the yoriginal'signal band incoming vfrom theline LNV isstepped up in the frequency spectrum-,bygtwonsteps ofmodulation and is transmitted.asasingle side band to the receivingstationgwhere fit is in turn-stepped by two steps,gf ,demodulai tion toits original vposition in the frequency spectrum. In a similar manner asignal band incoming from the line LE will be stepped up by thetransmitting apparatus symbolically indicated at 28 and radiated by thetransmitting antenna 2l to the receiving antenna 29' to be in turnstepped down bythe apparatus symbolically indicated at 30 and thenimpressed upon the line LV. Thus a two-way conversation may take placebetween the lines LW and LE- through the inediuinof the radio channels.

The foregoing description sets forth a" 4 type of radio telephone systemto which the apparatus may be applied for compressingv 'the volume rangetransmitted and for main` `taining the transmission equivalent of thealsystem. constantE inaccordance with the pres- "ent'invention, For thesepurposes a source of current haringafi'equency. just outside the 'voicerange, for enample,.3,000 cycles, is 'connected through V'a high-passfilter 3l f to the'potentiometen 1,3` associated with the'v amplifier l2of the vtransmitting channel connected to the line LW". yThis frequencyI`will be amplified lthe amplifierv 12 and y"subjected to anyl changeinA .volume-due to the'l adjustment of the 'potentiometer 131 `/wheneverit is adjusted to change the voli unief of they signal. transmitted.'This con- "tro'lling or pilot frequency willy bcapplied` toth'levarious.l modulators ,and radiated. in at Imanner similartothesignal, band `already described. .Ati thevreceiving channel assovciatedfwith the line L E va high-pass vfilterz `-32 similar tothe filterSlis bridged across i the channel ofthe output side'of'the lastdetectorDy for thepurpose of selecting the controlling or pilot. frequenc n Anampli# 'fier 33 is provided for ampli `ying the para 105 frequency toany desiredl value-before'im-V j l pressing yit upon the indicatingdevice. InU

orderI to produce indicationuuponi a direct indicatlng"` device byuneans 'of the pilot frequency, `it would be necessaryfocthe.

pilot frequency'tobe transanittedwvith con;

Lsiderable energy. ,Therefore .of the total" energy radiated, l

would 'have tolbe, assigned to the v. channel, thus rdecreasing `theamount o energy' available for thel transmissionof thel `a considerableportion ilot` signal. f Accordingly it is proposed by thev 'i presentinvention to transmit the pilot fre- .quency 'at a relatively lowtolume. and use'y `an aural receiving device such as a: telephone,.receiver Sil'connected to the ,output ofthe amplifier 33 for,indicating the -pilot frevquency. Asthe'telephone receiver will not giveva direct indicationofthc amplitudeofI the pilot' frequeney, a time.controlled interl `ruptcr 35 is providedin circuit with the' ceivingpoint la soul-ceci' alternating current' modulator will appear as Itheantenna 18'and Vreceived by volume necessary to y through a high 36having the same frequency as the pilot channel is connected through anadjustable potentiometer 37 to the terminals of the receiveg; 34. A timeoperatedinterrupter 38 ysimilar tothe interrupter 35 is included incircuit with the source 36, and this interrupter will be so timed as toapply `the frequency from the source 36 to the receiver 34 during .theinterval that the interrupter 35 cuts off the source 30.

The operation ofthe volumecompressing and transmission regulatingapparatus is as follows:

When the signal band incoming from the line LV is ldecreased in volumebelow the produce a full modulation, the decrease 1n volume will beindicated upon the volume indicator 14. The volume adjustingpotentiometer 13 will then be adjusted to increase the volume of thesignal to lthe value necessary to produce a complete modulation. Thepilot frequency from the source30 wil'l also be increased in volume bythe setting of modulate the carrier frequency of 33,000 cycles appliedto the modulator.. M. This a-3QO00 cycle component transmitted through"the'jffilter 16 which is then applied to the modllator M to modulatethev carrier frequency of 88,500 cycles. This modulation in turn appearsasva 58,500 cycle component transmitted thrpugh the filter 17 andimpressed upouHthe power amplifier P This component after beingamplified isradiatedu by the antenna 20. lItis thenimpressed upon thedetector D and b beating with the 90,000 cycle frequeney'ffom the source22 produces a component having the 'frequency of 31,500 cycles which is`passed .through the filter 23 and amplified by the amplifier 24. Theco1nponent is then impressed upon the detector D to beat with the 34,500cycle frequency from the source 25 thereby producing a component of3,000- cycles which is passed pass filter 32, amplified by the amplifier33 and impressed upon the telephonereceiver 34. Theattendant listeningin the receiver 34 will alternptely hear the control frequency of\3,000cycles\fr9m the source 30 andv a corresponding frequency of 3,000 cyclesfrom' the source 3(3ivhich is adjusted to some Jredetermined.amplitudewhich will givethe system a desired transmission equivalent.lhenever the.potentiometer 13 of the sending channel `is adjusted toincrease the volumeof the signal, the attendant will observe that. thevolume ofthev pilot frequency from the source 30 is greater than' thevolume vofthe 3,000 cycle frequency from the sourcel 36. `The attendantwill then adthe potentiometer and will vthe attendant heard 111 thereceiver the capacity of the various paratusA involved.' It will beclear, there-v quency from the 'source 30 until the .effect produced inthel receiver 34 from both sources is the same. A very accuratecomparison can be made of the volume transmitted from the two sources byreason of the fact that the noise picked up by the antenna 20 will beheard in the receiver 34 both when the pilot frequency from the source30 is being received and when the comparisonfrequency from the source 36is being received. The -adjustment of the potentiometer 25 to bring thetwo frequencies to the same volume results inan adjustment of the volumeof thesignal transmitted to the line LE which results in a decrease ofthe volume of the signal applied to the line I E which just compensatesfor the increase in volum'e due to the adjustment of the potentiometer13 at the transmitting channel. The over-all' transmission equivalent ofthe` radio channel is thereforethe same as before the adjustments weremade, and the signal is impressed upon the line LE with the same volumeit would have had in case no adjustment of transmission had been made atall. The transmission adjustments, however, result in signal atsubstantially its maximum value so that its transmission level duringtransmission through the ether will be well above the noise level.

Similarly, if the volume of the signal incoming from the line LlVoverload the vacuum tubes of the system, by observing either the volumeindicator 14 or the volume indicator 19 will be apprised of the factthat the volume -of the signalis greater than that previously vat thereceiving channel will accordingly adjust the potentiometer 25 so thatthe volume from both sources willbe the same. This result is an increasein the volume of the signal transmitted to theline LE, while theover-all transmisssiou equivalent of the radio channel will be aga-inresto-red to its normal value. The volume' of the signaltransmitted tothe line LE will -be ,as-great'as the volume would havebeenhad therebeen no transmission adjustment at all. At the same time the actualsignal the transmission of the is so great as to volume transmitted tothe various parts'ol the system .will not have been greater thanpieces'of apfore, that the signal impressed uponfthe line LE will 'rangein volun'ic over 'i'liojsanie range ol variation as the signal incomingfrom the liiieiluli, although 'ilie"actiia.l

voluiiie of thesignai triiiisiiiili'cd' fro'iii the antennalt ro.' theantenna 20 will be innintained ',faiil'yL constant. 'The rai'ige.` 'ofvolumev -actiiajlly lt'rimsniiited is therefor compressed to 'verynarroivliniits.

As is 'Wellknown"7 wlienfradio 's'igiials'are transmitted over longdistances considerable changefinzthey transmission ejicicncyiti'f themedii'iiii *occurs* from honi-"to hour duringthe day;I When anychangeccciirfs in the transmission jetliciency ofl thel ether 'in thesystem above described, this change 'will resiiltvin an increase or.decieasefin amplitude of the pilot: frequency impressed 'iipontlietendant'will"thereforeadjust the potentie# I meter 25 to bring anequality in volume be'- tWee-n the pilot frequency and the fre-` quencyfromithe rsource 3G@ This results in bringing 'the overlall'transmission. equivalent of'thte'ra-dio ,channel baclrfto -its normalValue. It'istlierefore clear that by means of the arrangements vabovedescribed, not, only 'may the Volume range `of the signal` actuallytransmitted 'be kept. within very narrow limits, 'but the over-all.transmis sion equivalent ofthe 'radio part of the system mayib'emaintained substantially constant. v.

Fig. 2"'illustrates a modification of the transmitting apparatuswhercbyth'e pilot fre uen'cy'fi's introduced as a radio frequency witout undergoing the scverahstep's, ot

modulation.` .In the case illustrated, 'the pilot frequency is generatedby the source 30 haring ,a frequency of 58,500 cycles.. This frequencyisiinpressed upon the chan-A uiel at a Ipoint beyond .the modulators,as,I

for example, upoii the Vinpu't side of the power amplifier PA. Thisavoids the ne- A cessity of 4passing the pilot. frequency heating 'withthclocal", Isources associatedk with the detectors D 'jan'd D, ofthe:receiving channel 'it will be vstepped down to 3,0001. cycles forselection bythe filter y32. This arrangement only permits of,adjustments` of volume in `tlierecciving channel to compensate Aforchanges iii the transmission etliciency of the medium. It does notindicate "ometer 13 uf'hieh controls the volume o f the 'of the signalactually radiated by the en frequency would be changed. The pilot f re-AI.synchronism lSince each interruptor aph plies theeorresponding sourceto the circuit i f for a period of, say, several seconds, absolute`syncliro.nism.. is not .necessn quency is i'iot subjected to suchvolume ,adjustinents.

,Fig hows an arrangement by which iliegpilot frequency may be `appliedto the transmitting channel at a`point beyondrtlie the medium. In thiscase the source l30 geny eratilng a frequency of 58,500 cycles 1s vcon-.nectedto the transmitting channel at the l same point as in Fig. 2.` Apotentiometer v 13', however., is interposed between thev source 30 andthe poweri amplifier P., l, This potentiometer is arranged to begad-` ,v

justed in the same manner 1as the potentisignal. Forexample, the Itwopotentiomef; t-ers may be made to move together asa unit by some Wellknown mechanical connection.

`lVith this arrangement, -when then poten-f tioineter 13 is adjusted tobring tlie volumey tenna 18 to its desired normal Value,'i1he\ I'adjustments ofthe volume of thel signal, i i `however,- for; the.reason thatl the pilot freliodulators and used to indicate change in i,t oliiine. at the transmitting channel as Well 'fas changes lin thetransmission elhciency of potentiometer 13 is given a. correspondingadjustment so that the volume of the pilot quency Will Vtherefore arrive,at ille-receiving j channel 'changed in volumel sol that theattendantobserving the ildifference in volume'. as'comparedqvitli thevolume `,from the source 36 may adjust the potentiometer 25.

4of the receivingl channel to compensate forv i.

the adjustment of the' potentiometer 13 of the transmitting channel?v"illierefore,` the `apparatusofalig. 3 cri-operating With the.Aapparatus of-,areceiving channel similar tolthat illustrated jinFig.=1,Y may be used not only to maintain the over-all transmission vequivalent ofthe radio part of the system constant, but it may be usedto keep lthel 'A yolume of the signal actually transmitted at or near adesired normal value, or, other, be used to compress thevolf Words, itmay urne range transmitted. I v f It `will be noted that the timecontrol vin.`

terrupters, such as 35 and 28, may be of any1 Well know-n type Acommonin the art, and nog difficulty Whatever need .be encountered inymaintaining =the.interrupters in propery iiiterrupter controlled by an Adirti niechanisniwill maintain the inter/Pup -thetwo distant points.,'fsilc'istantiiil'sy chronisin ifor a conside-table 1. periodIv 0f'-tiin without any.4 adjustiznent whatever. -1When'- ever .rit is.apparent `to. the? attendantmt theareceiving station that theinterrupters'are out of time, synchronism may be at once obtained bymerely adjusting lthe brush of many organizations widely different fromioy thosev illustrated without departing from the lowing claims:

' spirit of the invention as defined in the fol- What is claimed is:

\ l.- Inv a signaling system, a transmission medium, an adjustabletransmission element Aat the transmitting end of said medium,

'means to apply signaling current to said transmission element, means toadjust said transmission element in accordance with the variations involume of the signaling current, means to apply a control frequency tosaidfmedium, means to vary the volume y of said cont-rol frequency inaccordancewith the vadjustment of' said transmission element, acompensating transmission element 4at the' receiving station, means atsaid receiving station to receive said controlling frequency andvindicate changes in the volume thereof, and ymeans to adjust saidcompensating transmission element to compensate for the adjustment ofthe transmission element at-the sending station inA accordance with theindications produced by .said

Y controll frequency,

2. In a signaling; system, a'transmission medium an end of said medium,

means to impress signaling current upon said medium through ,means toadjust the gain of vsaid amplifier in accordance with the variavtionsinvolume of' said signaling current,

' Said medium,

means to impress controlling current upon means to vary the volume ofsaid controlling current in aceordancewith the adjustment of. thegain ofsaidv amplifier so that the controlling current will arfrive at thereceiving station with an amplitilde determined by the change in thegain y of theV amplifier' at the transmitting station, a v compensatingamplifier associated with said medium at the receiving station, meansreceive said conat. lthe receiving station to trolling current andindicate changesiin the amplitude thereof, and means to adjust`the'again of the compensating amplifier at the receiving station tocompensate for changes in the gain of the amplifier at the transmittingstation asjindicated by the changes in the volume of the receivedcontrolling current.

3. In a signaling system,l a transmission medium, an adjustabletransmission element associated therewith at the transmitting station,means-to impress signaling Icurrents amplifier at the transmitting ingstation in accordance with current, means to impress a controlling rui'-rent upon said medium, means to adjust lthe volume of saidcontrolling-current in accordance with the adjustment 4of saidtransmission element whereby .said controllingcuri'en't will arrive atthe receiving station with an amplitude determined by the adjustment ofsaid element and by any change in the transmission equivalent of saidmedium, a 'compensating adjustable transmission element associated withsaid medium at the receiving station', means at the receiving station toreceive said controlling current and indicate changes in the amplitudethereof,and means to adjust said compensating transmission element tocompensate for the changes in volume indicated by said controllingcurrent whereby the over-all transmission equivalent of the system willbe maintained, substantially constantfand the volume of the receivedsignal will be restored to its original valueuas compared'withthepre-determined normal value. 'i

4,-. In a signaling system, a transmissionA medium, an ampli erassociatedI with said medium at the transmitting station, means toimpress signaling currents upon said medium through. said amplifier,means to adjust the gain of said amplifier in accordance with the volumeof the signaling currents, means to impress a controlling currentuponpsa'id medium, means to vary the'V volume of said controllingcurrents in accordance with the adjustment of said amplifier wherebysaid controlling current lwill be transmitted over the medium and arriveat the receiving station with an amplitude determined by the change inthe gain ofsaid amplifier and .any vchange in the transmissionequivalent of said medium, a compensating amplifier associated with,said medium at the receiving station, means at the receiving station toreceive said controlling current and indicate changes in the volumethereof,y and means to adjust the gain of said compensating amplifier atthe receivthe indicated changes in volume of the received controllingcurrent whereby the over-all transmission equivalent of the systemwill"r be maintained substantially constant and the volume of thereceived signal will be restored to itsV original value as/ comparedwith the pi'e-detcrmined normal value.

5. In a radio signaling system, means at the transmitting station toradiate a signal, means at said station to radiate a pilot frequency,means lat the receiving station to receive and detect a'lradiatedsignal, an adjustable transmission element at the relcciving station foradjusting the volume of the received signal, 4means to select said pilotfrequency -at ,the receiving station, a' local source having the same:frequency as 1,565,544 Y l 'Z means to alternately impress the'seleetedby saidleomparison. pilot frequency and the frequency from the Intestimony whereof, I have signed my 10 local source -upon `theireceiverto compare name to this specification this 16th day of 5theirjimlun'les, and means to adjust said September, 1924.; l

transmission element at the receiving station inaccordance with thevariations in RALPH BOWN.

the pilot frequency at the receiving,r station; volume of' the pilotfrequency as indicated u

